Foamed resinous structures are useful in a wide variety of applications such as thermal insulation, in cushions, as packaging, and as adsorbents. Extruded foams are generally made by melting a polymer together with any desired additives to create a polymer melt. A blowing agent is mixed with the polymer melt at an appropriate temperature and pressure to produce a foamable gel mixture. The foamable gel mixture is then cooled and extruded into a zone of reduced pressure, which results in a foaming of the gel and the formation of the desired extruded foam product.
Nanoclays have been used to modify cellular foams in both batch and continuous extrusion foaming processes. It has been discovered that the addition of nanoclays into the polymer matrix of cellular foams results in an improvement in physical properties, such as improvements in mechanical strength (tensile modulus and strength and flexural modulus and strength), thermal stability, flame retardance, and barrier resistance. Three polymer/clay structures are possible: agglomerated nano-particles, intercalated nano-layers, and exfoliated nano-layers. In general, the greatest physical property enhancements, such as reducing the gas permeability, increasing the modulus, and/or increasing the thermal stability, are observed from nano-layers that are fully exfoliated throughout the polymer matrix.
Although nanoclays have a high aspect ratio, they require special treatment to separate the nano-layers and achieve exfoliation. One such manner of achieving exfoliated nanolayers is to incorporate an organic modifier in the clay. Layered nano-materials, such as organo-montmorillonite, typically contain approximately 30-40 wt % of low molecular weight organic modifiers. These modifiers are used to convert the clay surface from hydrophilic to hydrophobic, and to make the clay particles compatible with the polymer matrix for dispersion. Previous attempts to form an intercalated or exfoliated microstructure of a nanoclay-polymer matrix/composite have utilized modifiers such as methacryloyloxyethylhexadecyl-dimethyl ammoniums and methyl tallow bis-2-hydroxyethyl quaternary ammonia salts (e.g., Closite 20A, commercially available from Southern Clay Products Inc.). Although polymer nanoclay composite foams containing such modifiers have demonstrated promising fire retardant properties in terms of reduced heat release, smoke release rate, and char forming during firing, these surface modifiers are considered fire hazards based on industry fire tests, such as oxygen index and flame surface spread rates.
Therefore, there exists a need in the art to achieve an extruded or expanded polymer foam that contains a uniform clay dispersion in the polymer matrix that maintains the positive physical properties of nanoclay foam composites and meets the stringent requirements for industrial fire resistance applications.